Microlithography is used for the production of microstructured components, such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is referred to as a projection exposure apparatus having an illumination system and a projection objective. An image of a mask (=reticle) is illuminated via the illumination system, and the image of the mask is projected via the projection objective onto a substrate (for example a silicon wafer) which is coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection objective. This transfers the mask structure onto the light-sensitive coating on the substrate.
It is often desirable to produce light in the projection exposure apparatus that is as far as possible is unpolarized. For example, DE 198 29 612 A1 discloses depolarizing linearly polarized light from a laser source via a Hanle depolarizer and a light mixing system arranged downstream thereof.
In some instances, the light in the reticle plane of the illumination system can still have a residual polarization. Such residual polarization can be caused by anti-reflecting layers (AR layers) are present in the illumination system on the lenses, as well as the highly reflecting layers (HR layers) present on the mirrors.
It has been found that these effects can lead to non-homogenous distribution of the residual polarization. The non-homogenous distribution can be explained on the basis that a radial residual polarization distribution produced by the lenses (in particular the conical lenses of the axicon lens used in the illumination system) or by the AR layers on those lenses is superposed with a linear residual polarization of a constant preferred polarization direction, produced by the AR layers on the mirrors, wherein those mutually superposed residual polarization distributions increase or attenuate each other depending on the respective direction involved (for example perpendicular or parallel in relation to the scan direction).
US No 2005/0094268 A1 and WO 03/077011 A1 discloses breaking down an optical system into two subsystems with a retarder acting as a lambda/2 plate to be arranged therebetween. The retarder transposes two mutually perpendicular polarization states between the subsystems so that adding up the phase shifts in the second subsystem just cancels out that in the first subsystem. US No 2003/0086156 A1 discloses using a 90°-polarization rotator for example in a projection objective for mutual compensation of the retardations which are produced in a group which leads in relation to that 90°-polarization rotator and in a group which trails in relation to that 90°-polarization rotator.